Field of the Invention
The present invention relates to an alloy for an R-T-B-based rare earth sintered magnet and a manufacturing method thereof, and a manufacturing method of an R-T-B-based rare earth sintered magnet.
Priority is claimed on Japanese Patent Application No. 2015-236924, filed on Dec. 3, 2015, the contents of which are incorporated herein by reference.
Description of Related Art
Hitherto, R-T-B-based rare earth sintered magnets (hereinafter, sometimes simply referred to as an “R-T-B-based magnet”) have been used in motors such as voice coil motors in hard disc drives and motors for engines in hybrid vehicles or electrical vehicles.
R-T-B-based magnets can be obtained by molding and sintering R-T-B-based alloy powder primarily containing Nd, Fe and B. Generally, in R-T-B-based alloys, R refers to Nd or a substance containing Nd and other rare earth elements such as Pr, Dy and Tb that substitute some of Nd. T refers to Fe or a substance containing Fe and other transition elements such as Co and Ni that substitute some of Fe. B refers to boron, and some of B can be substituted by C or N.
The structure of an ordinary R-T-B-based magnet is mainly made up of a main phase made of R2T14B and an R-rich phase that is present in the grain boundary of the main phase and has a higher concentration of Nd than the main phase. The R-rich phase is also called a grain boundary phase.
In addition, generally, the composition of an R-T-B-based alloy is set so that Nd, Fe and B are in a ratio as close to R2T14B as possible in order to increase the proportion of the main phase in the structure of the R-T-B-based magnet (for example, refer to Masato SAGAWA, Permanent Magnet—Material Science and Application—, Pages 256 to 261 of Second Impression of the First Edition published on Nov. 30, 2008).
In addition, there are cases in which R-T-B-based alloys include an R2T17 phase. The R2T17 phase is known as a cause of the degradation of the coercive force or squareness of an R-T-B-based magnet (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2007-119882). Therefore, in a case in which the R2T17 phase is present in an R-T-B-based alloy, the R2T17 phase is removed in a sintering step of producing an R-T-B-based magnet.
In addition, since R-T-B-based magnets used in automobile motors are exposed to a high temperature in the motors, a large coercive force (Hcj) is required.
As a technique to improve the coercive forces of R-T-B-based magnets, there is a technique that substitutes Nd as R in an R-T-B-based alloy with Dy. However, Dy has biased resources and is thus produced only in a limited amount, and therefore it becomes difficult to stably supply Dy. As a result, studies are being made regarding techniques to improve the coercive force of an R-T-B-based magnet without increasing the amount of Dy contained in an R-T-B-based alloy.
In order to improve the coercive force (Hcj) of an R-T-B-based magnet, there is a technique that adds metal elements such as Al, Si, Ga and Sn (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2009-231391). In addition, it is known that Al and Si are incorporated into an R-T-B-based magnet as inevitable impurities as described in Japanese Unexamined Patent Application, First Publication No. 2009-231391. In addition, it is known that, when the amount of Si contained in an R-T-B-based alloy as an impurity exceeds 5%, the coercive force of an R-T-B-based magnet decreases (for example, refer to Japanese Unexamined Patent Application, First Publication No. H5-112852).
In the related art, there were cases in which it was not possible to obtain an R-T-B-based magnet having a sufficiently large coercive force (Hcj) even when metal elements such as Al, Si, Ga and Sn were added to an R-T-B-based alloy. As a result, it was necessary to increase the concentration of Dy even when the metallic elements were added.
The present inventors have studied the composition of the R-T-B-based alloy, and thus, have found that the coercive force reached the maximum at a specific concentration of B. Then, on the basis of the obtained result, the present inventors have succeeded in development of an R-T-B-based alloy which is completely different from the R-T-B-based alloy of the related art, from which an R-T-B-based magnet having a high coercive force can be obtained even in a case where the content of Dy contained in the R-T-B-based alloy is zero or is extremely small (refer to Japanese Patent No. 5613856 and Japanese Patent No. 5744286). The B concentration of the alloy is lower than that of the R-T-B-based alloy of the related art.
An R-T-B-based magnet manufactured by using the R-T-B alloy includes: a main phase that contains R2Fe14B as a main component; and a grain boundary phase that has a higher R content than the main phase, in which the grain boundary includes a grain boundary phase (transition metal-rich phase) having a lower rare earth element concentration (except for a grain boundary phase (R-rich phase) which is conventionally known to have a high rare earth element concentration) and a higher transition metal element concentration than a grain boundary phase of the related art. An R-T-B-based magnet of the related art includes: a main phase as a magnetic phase that exhibits coercive force; and a grain boundary phase as a non-magnetic phase that is disposed in grain boundaries of the main phase. It is considered that, in the new R-T-B-based magnet developed by the present inventors, the transition metal-rich phase includes a large amount of transition metal and thus exhibits coercive force. The magnet in which the phase exhibiting coercive force (“transition metal-rich phase”) is present in the grain boundary phase is revolutionary enough to defy past common knowledge.
The R-T-B-based magnet belongs to a composition range in which a concentration of boron (B) is lower than that of a theoretical composition of R2T14B, and can be manufactured by using an alloy to which a trace amount of a metal element is added. Hereinafter, the R-T-B-based magnet may be referred to as an R-T-B-based magnet containing a low amount of boron.